Catalytic reactor

ABSTRACT

Disclosed is a catalytic reactor in which a plurality of pipe-shaped catalysts are arranged, said reactor including a catalyst-supporting structure which is capable of varying the distance between the adjacent catalysts. Disclosed is also a method for constructing such a reactor, said method comprising optimally setting the distance between the adjacent catalysts from one operation to another.

TECHNICAL FIELD:

The present invention relates to a catalytic reactor and moreparticularly to a catalytic reactor utilizing pipe-shaped catalysts anda method for constructing the same, the reactor being suitable for use,for example, as a reactor for dry denitrification.

BACKGROUND ART:

Although a variety of systems for dry denitrification have been hithertoproposed, most of them utilize a moving-bed reactor in which particulatecatalysts are used. This is because a packed-bed reactor utilizingparticulate catalysts has a disadvantage that the catalyst bed isblocked by foreign materials such as dust which are contained in the gasto be treated. As one way for overcoming such a disadvantage, it wasproposed to use a "dust-free" catalyst. However, dust-free catalystshave not been put into a practical use since most of the dust-freecatalysts do not yet exhibit a sufficient resistance to dusts andfurther, dust-free catalysts generally have a low catalytic surface areaper volume and hence need to be loaded in a larger amount as comparedwith particulate catalysts.

However, along with development of more active dust-free catalysts, adenitrification device utilizing dust-free catalyst has been recentlyrecognized because of its structural simplicity and easiness ofoperation. There are a variety of shapes of dust-free catalysts, sometypical examples being pipe-shaped catalyst, honeycomb-shaped catalyst,plate-shaped catalyst and so on. Among these, pipe-shaped catalyst isthe most advantageous, as compared with the other dust-free catalysts,particularly with honeycomb-shaped catalyst, because of its lower costsince it is easy to be shaped, has low loss of catalyst material inshaping and further, can be manufactured with the use of a relativelysmall-scale device. With the recent enlargement of dry denitrificationsystems, the cost of the catalyst and the cost of the reactor have cometo contribute more to the overall cost of the system. Therefore, apipe-shaped catalyst, because of its low cost, is suitable for useparticularly in a large-scale device for dry denitrification.

Although pipe-shaped catalyst is highly rated in the economical sense asstated above, it has disadvantages due to the arrangement thereof in thereactor. As seen from FIGS. 1(A) and (B) showing partial cross sectionalviews of reactors, pipe-shaped catalysts have been hitherto arranged inreactors in the square closest packing mode (A) or triangular closestpacking mode (B). The gas to be treated is to pass through flow passagesas defined by the inner surfaces of the pipe-shaped catalysts (thepassages designated as 1 or 1') and also through flow passages asdefined by the outer surfaces of the pipe-shaped catalysts (the passagesdesignated as 2 or 2'). However, the flow passages as defined by theinner surfaces and the flow passages as defined by the outer flowpassages are greatly different from each other in their cross sectionalshapes as seen from FIGS. 1 (A) and (B), and hence, the states of thegas flowing through such two types of passages are greatly different.This will be clear from the following fact.

FIG. 2, FIG. 3 and FIG. 4 show the results of fluid dynamic experimentscarried out by the present inventors, for the case where pipe-shapedcatalysts, each having an inside diameter of 21 mm and an outsidediameter of 32 mm, are arranged in the square closest packing mode shownin FIG. 1(A).

The graph of FIG. 2 shows the relationship between the Reynolds number(Re) and the friction factor (f) of the gas flowing in the flow passagesas defined by the inner surfaces of the pipe-shaped catalyst. The graphof FIG. 2 clearly shows the characteristic of turbulent flow in arough-wall pipe, as is well known in fluid dynamics.

The graph of FIG. 3 shows the relationship between the Reynolds number(Re) and the friction factor (f) of the gas flowing in the flow passagesas defined by the outer surfaces of the pipe-shaped catalysts. From FIG.3, it is understood that the gas flow is laminar in the outer surfaceflow passages.

FIG. 2 and FIG. 3 show the results of experiments made with the samepressure drop across the outer surface flow passages and across theinner surface flow passages. In a practical reactor for drydenitrification, the pressure drop across the outer surface flowpassages and that across the inner surface flow passages are equal.Thus, from the experimental data as shown in FIG. 2 and FIG. 3, it isunderstood that, in a reactor in which the catalysts are arranged asshown in FIG. 1 (A), the gas flows in turbulent flow through the innersurface flow passages and in laminar flow through the outer surface flowpassages.

FIG. 4 shows the experimental results (those as shown in FIG. 2 and FIG.3) in terms of the relationship between linear velocity of the gasflowing through the passages and pressure loss per meter of the lengthof the passages. From FIG. 4, it is understood that for yielding a givenpressure loss, the gas velocity for the inner-surface flow passages(graph 5) and that for the outer-surface flow passage (graph 6) are muchdifferent from each other. Further, since the cross-sectional area of aninner-surface flow passage is approximately 346 mm² and the crosssectional-area of an outer-surface flow passage is approximately 218mm², the difference in terms of the gas flow rate (linear velocitymultiplied by cross sectional area) is greater.

As shown in FIG. 2 through FIG. 4, although the total catalytic surfacearea of the outer-surface passages are larger than that of theinner-surface flow passages, the gas flows through the outer-surfaceflow passages at a smaller flow rate and further in a laminar flow. Itcan be therefore said that the outer-surface flow passages are notsufficiently utilized as catalyst and, from the stand point of efficientutilization of the catalyst, there is too much loss in such conventionalmodes of catalyst arrangement as the square closest packing arrangementmode.

Furthermore, in a case where the gas velocity is extremely low locallyin the flow passages, there is a risk that the dust contained in the gaswill accumulate so as to lead the blockage of the flow passages inaddition to the lowering of catalytic activity. As a matter of fact, itwas confirmed through the experiments of the present inventors that suchdust accumulated in the circumferential portions in the outer-surfaceflow passages.

As a way for overcoming the above-mentioned disadvantages, i.e., forequalizing the flow states of the gas in the two types of flow passages,it may be proposed to make the shapes of the two passages to behydro-dynamically similar and more particularly, as a concrete andsimple answer, to arrange pipe-shaped catalysts so that the hydraulicdiameters of the two passages are equal. This solution is based on ahydrodynamic assumption that the friction factor for all of the flowpassages are equal. However, as seen from FIG. 2 and FIG. 3 based on theexperiments of the present inventors and showing that the frictionfactor for the outer-surface flow passages is different from frictionfactor for laminar flow in a cylindrical tube, when the geometricalshapes of flow passages are different, the friction factors for thepassages are different. In addition, as will be explained later, thefriction factor is closely related to the catalytic reaction rateconstant. Therefore, from the standpoint of catalytic reactionengineering, such a determination of the distance between the catalysts,in disregard of the difference in friction factors, will not achieve anypractical effects.

DISCLOSURE OF THE INVENTION

Accordingly, the principal object of the present invention is, for acatalytic reactor in which a plurality of pipe-shaped catalysts arearranged, to make more efficient use of the catalysts through morestrict analysis from the stand point of reaction engineering than thosehitherto made.

After strenuous studies, the present inventors have found that, for moreefficient use of a catalytic reactor in which a plurality of pipe-shapedcatalysts are arranged, the cross-sectional area of the outer flowpassages as defined by the spaces between the outer surfaces of thepipe-shaped catalysts should be varied depending upon the operationconditions to be employed and the catalytic activity, rather than in theconventional modes of catalyst arrangement such as the square closestpacking or triangular closest packing modes, and further that thedistance between adjacent pipe-shaped catalysts should be set so thatthe flows of the reactant gas are in turbulent flow not only in theinner flow passages as defined by the inner surfaces of the pipes butalso in the outer flow passages and also so that the followingrelationship is satisfied:

    K.sub.1 (S.sub.1 /G.sub.1)=K.sub.2 (S.sub.2 /G.sub.2)      (1)

where

G₁ is flow rate of the reactant gas through each of the inner flowpassages;

G₂ is flow rate of the reactant gas through the outer flow passage perpipe-shaped catalyst,

S₁ is cross-sectional area of each of the inner flow passages,

S₂ is cross-sectional area of the outer flow passage per pipe-shapedcatalyst,

K₁ is apparent reaction rate constant with respect to the catalyticreaction occuring in the inner flow passages,

K₂ is apparent reaction rate constant with respect to the catalyticreaction occuring in the outer flow passages.

Thus, according to the present invention, there is provided a catalyticreactor in which a plurality of pipe-shaped catalysts are arranged inparallel with each other and in which a catalytic reaction proceedswithin the flow passages as defined by the inner surfaces of saidpipe-shaped cataysts and also within the flow passages as defined by thespaces between the outer surfaces of said pipe-shaped catalysts,characterized in that said reactor includes a catalyst-supportingstructure for supporting said pipe-shaped catalysts which is capable ofvarying the distance between adjacent pipe-shaped catalysts, whereby thecross-sectional area of said flow passages as defined by the spacesbetween the outer surfaces of the pipe-shaped catalysts are optimallyset depending upon the operation conditions of the reactor.

Further, according to the present invention there is provided a methodfor constructing the abovementioned catalyst, characterized in that thedistance between adjacent pipe-shaped catalysts is set in accordancewith calculated value of the hydraulic diameter of the outer flowpassage per pipe-shaped catalyst, the calculation being made forconditions under which the flows of the reactant gas are turbulent inboth said inner flow passages and said outer flow passages and thefollowing relationship is satisfied:

    K.sub.1 (S.sub.1 /G.sub.1)=K.sub.2 (S.sub.2 /G.sub.2)

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 (A) and (B) are partial cross sectional view of conventionalcatalytic reactors and show arrangements of pipe-shaped catalysts.

FIG. 2, FIG. 3 and FIG. 4 graphically show the results of fluid dynamicexperiments achieved by the present inventors, for a case where thecatalysts are arranged as shown in FIG. 1 (A).

FIG. 5 graphically shows the relationship between friction factor andReynolds number for the outer flow passages of the pipe-shaped catalyst,which is necessary in the practice of the invention.

FIG. 6 shows the relationship between gas linear velocity and apparentreaction rate constant for the case where the reactant gas flows throughthe inner flow passages of the pipe-shaped catalysts, which is necessaryin the practice of the present invention.

FIG. 7 is a partial cross-sectional view of an example of the reactoraccording to the present invention.

FIG. 8 and FIG. 9 graphically show the results of experiments ondenitrification reaction with the use of the reactor of the presentinvention, in comparison with those with a conventional reactor.

BEST MODE FOR CARRYING OUT THE INVENTION

With respect to a reactor in which a suitable number of pipe-shapedcatalysts having predetermined inside and outside diameters are arrangedin parallel at equal distances, it can be approximated that, to each ofthe pipe-shaped catalysts, except for the ones arranged in the vicinityof the reactor wall, there are imparted one inner flow passage ofequivalent cross sectional area and one outer flow passage of equivalentcross sectional area. For a catalytic reactor, such as practical devicefor dry denitrification, in which a great number of pipe-shapedcatalysts are arranged, the error due to such approximation isnegligible.

Thus, in Eq. (1) in the above, the cross-sectional area of each of theinner flow passages, S₁, can be expressed as π(D₁ /2)² where D₁ is theinside diameter of the pipe-shaped catalyst, the cross-sectional area ofthe outer flow passage per pipe-shaped catalyst, S₂, as L₂ D₂ /4 whereL₂ is the length of reactant gas-catalyst contact in said outer flowpassage and D₂ is the hydraulic diameter of the flow passage perpipe-shaped catalyst, the flow rate of the gas through each of the innerflow passages, G₁, and S₁ u₁ where u₁ is the linear velocity of thereactant gas through each of the inner flow passages, and, the flow rateof the gas through the outer flow passage per pipe-shaped catalyst, G₂,as S₂ u₂ where u₂ is the linear velocity of the reactant gas through theouter flow passages per pipe-shaped catalyst.

Accordingly, if the reaction rate constant K₁ can be expressed in termsof u₂ and the reaction rate constant K₂ can be expressed in terms of u₂and D₂, then, Eq. (1), which is for a catalytic reactor in whichpipe-shaped catalysts of predetermined inside diameter D₁ are arranged,can be expressed in terms of u₁, u₂ and D₂.

It is said that the catalytic reduction reaction of nitrogen oxides withammonia in a first-order reaction with respect to the concentration ofthe nitrogen oxides in the gas to be treated. For a case, asdenitrification reaction just mentioned, where the reaction is afirst-order reaction with respect to the concentration of reactant andthe reaction proceeds in the vicinity of the outer surface of thecatalyst, it is known that apparent reaction rate constant can be givenby the following equation:

    1/K=(1/K.sub.a)+(S/L)(1/K.sub.f A)                         (2)

where

K_(a) =reaction rate constant with respect to catalyst surface,

K_(f) A=film coefficient of mass transfer,

S=cross sectional area of flow passage,

L=length of gas-catalyst contact.

In Eq. (2), Ka is intrinsic to the properties of the catalyst per seincluding its catalytic activity and KfA is mainly determined by thestate of gas flow.

For the reaction occuring in the inner flow passages and that in theouter flow passages, respectively, the following equations apply:

    1/K.sub.1 =(1/K.sub.a)+(S.sub.1 /L.sub.1)(1/KfA.sub.1)     (2')

    1/K.sub.2 =1/K.sub.a +(S.sub.2 /L.sub.2)(1/KfA.sub.2)      (2")

Hereinafter, subscripts 1 and 2 indicate conditions for the inner flowpassages and for the outer flow passages, respectively. KfA and thestate of gas flow can be correlated to each other, for example, by thefollowing equation:

    Jd=(KfA/u)(Sc).sup.2/3 =1/2f                               (3)

where Jd is Chilton-Colburn's mass transfer factor, Sc is the Schmidtnumber, f is the friction factor and u is the linear velocity of gas.Eq. (3) is an empirical equation supported by a number of experiments,and further it has been confirmed that the relation expressed by thisequation is applicable regardless of whether the gas is in turbulentflow or in laminar flow and regardless of the geometrical shape of flowpassage.

The pressure loss across a catalyst bed, ΔP, is given by

    ΔP.sub.1 =2f.sub.1 u.sub.1.sup.2 ρ/D.sub.1 g.sub.c (4)

    ΔP.sub.2 =2f.sub.2 u.sub.2.sup.2 ρ/D.sub.2 g.sub.c (5)

where f₁ and f₂ are the friction factors, D₁ and D₂ are the hydraulicdiameters, u₁ and u₂ are the gas linear velocities, ρ is the gas densityand g_(c) is the gravitational conversion factor (9.8 m/sec²).

It is known that the friction factor, f₁, f₂, can be expressed as afunction of Re (Reynolds number) by means of experiments. For example,f₁ is empirically expressed as shown by the straight line of FIG. 2 by

    f.sub.1 =0.0304 Re.sub.1.sup.-0.09                         (6)

where 4000<Re₁ <12,000.

According to the present invention, pipe-shaped catalysts are arrangedin a reactor so that the gas is to flow in turbulent flow not only inthe inner flow passages but also in the outer flow passages. Foraccomplishing this, in the practice of the present invention, thefriction factor for the outer flow passages, f₂, is empiricallypredetermined in terms of Reynolds number for a certain range ofhydraulic diameter of the flow passage, D₂. FIG. 5 shows an example ofsuch empirical data, and it is seen therefrom that f₂ is expressed inthe terms of Reynolds number (Re₂) by the following equation

    f.sub.2 =0.0553 Re.sub.2.sup.-0.154                        (7)

where

4500<Re₂ <15000

20×10⁻³ (m)<D₂ <40×10⁻³ (m)

For simplifying Eqs. (2') and (2"), it is necessary to obtain the valuesfor Ka and Sc, as understood with reference to Eq. (3). Thus, as aconvenient way for obtaining the values for Ka and Sc, the values forapparent reaction rate constant are utilized by the present inventors,the values having been obtained beforehand by reaction procedure whichis conducted by passing the reactant gas through only the inner flowpassages defined by the pipe-shaped catalysts. For example, FIG. 6 showsrelationship between the gas linear velocity and the reaction rateconstant for dentrification reactions which were conducted at thetemperature of 350° C. by passing the reactant gas through only theinner flow passages of the pipe-shaped catalysts.

From Eqs. (3) and (6), KfA₁ is given by

    KfA.sub.1 =au.sub.1.sup.0.91                               (8)

where a is an unknown proportion constant.

Thus, for example, since, from the data of the activity test at 350° C.

at u₁ =3 (m/sec) K₁ =8.63

at u₁ =20 (m/sec) K₁ =7.52,

substitution of these values in Eq. (2) yields ##EQU1##

By the solution of Eqs. (9) and (10), the values for Ka and Sc areobtained as follows.

Ka=12.9 (1/sec)

Sc=1.7056

By putting the values obtained, Eq. (2') becomes ##EQU2## where5.25×10⁻³ is the value for S/L of the inner flow passage.

The curved line in FIG. 6 shows the relationship between gas linearvelocity u₁ (m/sec) and denitrification reaction rate constant K₁ in theinner flow passage of the pipe-shaped catalyst, and is in a closeagreement with the values obtained previously by the activity test whichare shown by the dots in the same figure.

Since the properties of the catalyst, including catalytic activity, arethe same both for the outer flow passages and the inner flow passages,the value for Ka as obtained in the above-mentioned manner can beutilized in obtaining K₂, the reaction rate constant for the outer flowpassages. In addition, as a convenient way, it is assumed that the valuefor Sc which has been obtained for the inner flow passages is applicableto the outer flow passages as well. Thus, from Eqs. (2"), (3) and (7),the apparent reaction rate constant with respect to the reactionoccurring in the outer flow passages is given by the following equation##EQU3##

Thus, K₁ can be expressed in terms of u₁ and K₂ can be expressed interms of u₂ and D₂.

Since in a practical reactor the gas flows so that the pressure loss ΔPacross the inner flow passages and that across the outer flow passagesare equal, ΔP₁ =ΔP₂. Therefore, by substitution of Eqs. (6) and (7) forfriction factor in Eqs. (4) and (5), respectively, u₂ can be expressedin terms of u₁ and D₂ as given by the following equation

    u.sub.2 =9.9675 u.sub.1.sup.1.0347 D.sub.2.sup.0.6251      (13)

In the manner above stated, K₁ can be expressed in terms of u₁, and K₂can be expressed in terms of u₁ and D₂. Therefore, for a catalyticreactor of predetermined L₁, L₂, D₁ and S₁, Eq. (1) finally gives therelation between u₁ and D₂.

For example, in the case above exemplified, in which

L₁ =0.066 m

L₂ =0.1005 m

D₁ =21×10⁻³ m

S₁ =3.463×10⁻⁴ m

Eq. (1) finally becomes

    u.sub.1 (0.07752=0.347 u.sub.1.sup.-0.91)=9.968D.sub.2.sup.0.6251 u.sub.1.sup.1.0347 (0.07752+8.44s D.sub.2.sup.0.6251 u.sub.1.sup.-0.8753) (14)

Some examples of calculation in accordance with Eq. (14) are tabulatedbelow.

    ______________________________________                                        u.sub.1                                                                       (m/sec) 9.2     9.5    10    10.5 11    11.5 12                               ______________________________________                                        D.sub.2                                                                       (mm)    22.23   22.30  22.24 22.18                                                                              22.12 22.07                                                                              22.02                            ______________________________________                                        u.sub.1 13      14     15    16   17    18   20                               ______________________________________                                        D.sub.2 21.92   21.83  21.74 21.67                                                                              21.57 21.53                                                                              21.40                            ______________________________________                                    

If D₂, the hydraulic diameter of the outer flow passage per pipe-shapedcatalyst, is obtained by the calculation in the above manner, then it iseasy to obtain the value for the distance between the centers of thepipe-shaped catalyst as will be described below, and the pipe-shapedcatalysts may be arranged in the reactor in accordance with such value.

For example, in the case where a plurality of pipe-shaped catalyst areto be arranged at equal distances in a triangular arrangement as shownin FIG. 7, the cross sectional area of the outer flow passages, S₂, isgiven by the following equation. ##EQU4## where x is the distancebetween the centers of the pipe-shaped catalysts and d₂ is the outsidediameter of the pipe-shaped catalyst.

Since S₂ =L₂ D₂ /4, Eq. (15) becomes ##EQU5##

Accordingly, in the case where L₂ =0.1005 (m) and d₂ =32×10⁻³ (m), ifthe calculated value of D₂ is 21.74×10⁻³ (m) at 15 m/sec, it followsfrom Eq. (16) that:

x=0.039 (m)

Furthermore, it was found by the present inventors that in the actualarrangement of the catalysts, strict contentment of the value for D₂with Eq. (1) is not always needed, but, satisfactory results will beobtained if the hydraulic diameter is so determined that it falls withinthe range of 0.9a to 1.25a where a is the value for D₂ calculated fromEq. (1).

Therefore, according to the present invention, the catalysts areoptimally arranged from one operation to another, depending upon thelinear velocity of reactant gas or flow rate of reactant gas employed.

When the pipe-shaped catalysts are supported at a suitable distance andthe distance between the catalysts are so set that the above equation(1) is satisfied and further the flows of the gas are in turbulent flowin both the inner and outer flow passages in accordance with the presentinvention, there are obtained such effects as listed below:

(a) The amount of catalysts can be reduced in comparison with theconventional arrangement of closest catalyst packing.

(b) The size of the reactor can be reduced in comparison with thearrangement of closest catalyst packing.

(c) There are now flow passages where the gas flow rates are retarded,so that the accumulation of dust is prevented.

(d) The pressure loss can be reduced in comparison with the closestcatalyst packing arrangement.

The above effects are confirmed by the following examples.

FIG. 7 is a partial cross sectional view of an embodiment of the reactorof the present invention. In this embodiment, the catalyst-supportingstructure of the reactor comprises partition plates 18 which separatethe pipe-shaped catalyst from each other and support said catalysts. InFIG. 7, 10 is the surrounding wall of the reactor, 11 (the region ashatched) is the outer flow passage per pipe-shaped catalyt, 12 is theinner flow passage, 13 is D₁, and 19 is x, the distance between thecatalysts to be obtained from the hydraulic diameter D₂. The results areshown in FIG. 8 and FIG. 9 on the experiments of denitrificationreaction with the reactor as shown in FIG. 7, in comparison with thosewith a reactor in which pipe-shaped catalysts, having the same outsideand inside diameters, are arranged in the closest square packing mode.The temperature was 340° C.

The apparent reaction rate constants (K) as shown in FIG. 8 are not theones with respect to each of the pipe-shaped catalysts, but the oneswith respect to the overall volume of the reactors. LV signifies spacevelocity of the gas in terms of the normal conditions. As seen from thisfigure, the apparent reaction rate constants with respect to the overallreactor volume, those as shown by curved line 15, for the case where thecatalysts are optimally arranged in accordance with the presentinvention in the triangular arrangement with the distance between thecenters of the catalysts being 37 mm, are substantially equal to those,as shown by the curved line 14, for the case of the square closestpacking arrangement. Thus, it was appreciated that the amount of thecatalysts for the same total volume of reactor is reduced, through theoptimal arrangement of the catalyst of the present invention, byapproximately 20% as compared with the square closest packingarrangement.

FIG. 9 shows pressure loss per meter of the reactor length, ΔP, againstLV. Straight line 16 shows the values for square closest packing, whilestraight line 17 shows the values for the optimal arrangement in thestaggered mode in accordance with the present invention where thedistance between the centers of the catalysts is 37 mm. The values forpressure loss as shown by FIG. 9 are not the ones for the catalystsonly, but the ones for the supporting structure in addition to thecatalysts. From FIG. 9, it is seen that, for the same value of spacevelocity, the pressure loss can be reduced by approximately 30% throughthe optimal setting, in accordance with the present invention, of thedistance between the supported catalysts.

According to the present invention, the distance between pipe-shapedcatalysts can be readily calculated depending upon a predetermined flowvelocity of reactant, in the manner as described above. In addition, theabove equations can also be solved simultaneously with equation(s) forexpressing the reaction rate in the reactor. In such a manner, Eq. (1)can be expressed finally in terms of D₂ and the conversion ratio of thereactant, and thus, the distance between the catalysts may be setdepending upon a predetermined conversion ratio of the reactant.

Although the present invention has been described mainly with respect toa reactor for dry denitrification, it should be noted that the inventionis applicable to any other catalytic reaction process in which a similarreactor is employed and the reaction proceeds according to a similarmechanism to that for the denitrification reaction.

We claim:
 1. A catalytic reactor in which a plurality of pipe-shapedcatalysts are arranged in parallel with each other and in which acatalytic reaction proceeds within the flow passages as defined by theinner surfaces of said pipe-shaped catalysts and also within the flowpassages as defined by the spaces between the outer surfaces of saidpipe-shaped catalysts, characterized in that said reactor includes acatalyst-supporting structure for supporting said pipe-shaped catalystswhich is capable of varying the distance between adjacent pipe-shapedcatalysts, whereby the cross-sectional area of said flow passages asdefined by the spaces between the outer surfaces of the pipe-shapedcatalysts is optimally set depending upon the operation conditions ofthe reactor by arranging the pipe-shaped catalysts so that the followingrelationship is satisfied:

    K.sub.1 (S.sub.1 /G.sub.1)=K.sub.2 (S.sub.2 /G.sub.2)

where S₁ =cross sectional area of each of the inner flow passages, S₂=cross sectional area of the outer flow passage per pipe-shapedcatalyst, G₁ =flow rate of the reactant gas through each of the innerflow passages, G₂ =flow rate of the reactant gas through the outer flowpassage per pipe-shaped catalyst, K₁ =apparent reation rate constantwith respect to the catalytic reaction occurring in the inner flowpassages,and K₂ =apparent reaction rate constant with respect to thecatalytic reaction occurring in the outer flow passages.
 2. A method forconstructing a catalytic reactor which comprises arranging a pluralityof pipe-shaped catalysts of predetermined inside and outside diameters,in parallel with each other, so that the inner surfaces of saidpipe-shaped catalysts define inner flow passages and the spaces betweenthe outer surfaces of said pipe-shaped catalysts define outer flowpassages in order to feed reactant gas into said inner flow passages andsaid outer flow passages, characterized in that the distance betweenadjacent pipe-shaped catalysts is set in accordance with a calculatedvalue of the hydraulic diameter of the outer flow passages perpipe-shaped catalyst, the calculation being made for conditions underwhich the flows of the reactant gas are turbulent in both said innerflow passages and said outer flow passages and the followingrelationship is satisfied:

    K.sub.1 (S.sub.1 /G.sub.1)=K.sub.2 (S.sub.2 /G.sub.2)

where S₁ =cross sectional area of each of the inner flow passages, S₂=cross sectional area of the outer flow passage per pipe-shapedcatalyst, G₁ =flow rate of the reactant gas through each of the innerflow passages, G₂ =flow rate of the reactant gas through the outer flowpassage per pipe-shaped catalyst, K₁ =apparent reaction rate constantwith respect to the catalytic reaction occurring in the inner flowpassages,and K₂ =apparent reaction rate constant with respect to thecatalytic reaction occurring in the outer flow passages.